The invention relates to a photoconductor-based electrical switch.
Photoconductor-based electrical switching consists of the connection of two circuit elements by a semiconductor material that is highly insulating in darkness and in setting up a connection by subjecting this solid to a light flux that makes it conductive. The conduction thus results from the photo-creation of electrically charged carriers (electrons and holes).
Photoconductor-based electrical switching has a number of advantages as compared with other modes of control (such as mechanical modes, electrical arcs and discharge in gases, etc.).
These advantages are:
the galvanic insulation of the actuator, PA1 a very short intrinsic response time that corresponds to the laser pulse used, PA1 the absence of fluctuations at the instant at which the current is set up, determined by the appearance of light.
The optical control source is represented by a laser. At present, ultra-short optical pulses (of some 10.sup.-15 s) are obtained from such sources. This makes it possible to obtain electrical pulses with very sudden transitions.
In the case of high-power electrical pulses (for example in the range of 1 MW), the photoconductor is subjected to high currents (kA). It is then desirable to have a wide-sectioned photoconductor (for example of some mm.sup.2). Furthermore, the selection switching of high voltages (kV) implies the use of switch structures of sufficient length (some mm) in order to avoid reaching the zone of rupture of the material. All the sizes are in the range of 1 mm (or more) and it is desired to create charge carriers in the entire volume.
The optical absorption of semiconductor materials at a given wavelength is generally either very low (the energy of the photon is below that of the bandgap energy of the material) or very high (with an absorption coefficient of several hundreds of cm.sup.-1 once the energy of the photon is greater than that associated with the bandgap). The search for intermediate values of the absorption coefficient requires a relatively precise matching between the excitation wavelength (laser) and the threshold wavelength of the semiconductor (associated with the bandgap energy). This matching is generally not obtained.
Furthermore, since the absorption is very high, it is done generally on the surface of the material. The thickness of ionized material is then very small. The section of the material perpendicular to the switch-over direction and therefore perpendicular to the flow of a current to be switched over is very small. If the current to be switched over has very high intensity, there is a risk that the material might be damaged in the absorption zone owing to the heating caused by the passage of the current.
Certain semiconductors such as silicon have absorption coefficients that are lower at the wavelengths below the absorption threshold (as compared with GaAs). These semiconductors constitute a limitation on high-power switch-over operations. Furthermore, the characteristics of the carriers in these semiconductors do not favor the obtaining of efficient switching (low mobility of carriers).